Changes

SPIGOT-4706: Can't interact with active item

Remove the Damage tag from items when it is 0.
CraftBukkit (and Minecraft as well in some cases, such as when getting 
an item from the creative inventory menu) will omit the Damage tag when 
it is zero. However, minecraft will add the tag in some situations 
nevertheless, such as when loading the ItemStack, or when explictly 
setting the item undamaged.
These items (with and without the Damage tag for undamaged items) will 
be considered as different by minecraft and CraftBukkit in various 
situations, even though they should not. In CraftBukkit these items will 
actually only be considered unsimilar if the items' metadata is not
'empty' (if it contains other additional metadata, such as enchantments, 
etc.).
If the item's tag is empty after removing the Damage tag, it gets 
completely removed. The setRepairCost function was adapted to behave in 
the same way (removal of the tag if it becomes empty).